The present invention relates generally to non-destructive testing, and, more specifically, to eddy current inspection.
Gas turbine engines include rotating shafts and disks which support rotating blades in the fan, compressor, high pressure turbine, and low pressure turbine. Commercial and military turbine engines used for powering aircraft in flight require minimum weight while still ensuring a suitable useful life of the engine components.
The rotating components are subject to substantial centrifugal loads during operation which generate corresponding stress that must be limited for maximizing component life. Various forms of superalloy materials are commonly used in modern aircraft turbine engines for ensuring component integrity over the useful life thereof.
However, defects, flaws, or other anomalies in the material may be introduced during the original manufacture of the engine components, or may occur during the operational life thereof. Accordingly, the engine components are typically inspected during the manufacturing process, and during routine maintenance outages, for uncovering any anomaly therein which might limit the useful life of the components.
A common, non-destructive inspection technique is eddy current (EC) inspection of typically metal components. An EC probe includes a small electrical coil mounted near the tip thereof through which an alternating current is generated, which in turn produces an eddy current in the component. The probe tip is moved along the surface of a component for inspection and is used to measure the interaction between the electromagnetic field and the component. A defect or geometric abnormality in the material which changes the homogeneity thereof will disturb the eddy current. The disturbed eddy current modifies the exciting current in the probe coil, and the modified current is then suitably detected and correlated to particular properties of the material to indicate the corresponding anomaly.
For example, eddy current inspection is commonly used for measuring residual stress, density, and degrees of heat treatment in typically metal components. It is also typically used for detecting physical defects or abnormalities on or near the material surface such as dents, bumps, or minute cracks in the material.
Crack detection is particularly important in turbine engine components since cracks may propagate under stress and substantially reduce the useful life of a component, and may eventually lead to component failure if not suitably accommodated.
The electrical coil in a typical eddy current probe is relatively small, for example, about 0.5 mm in diameter for ensuring high sensitivity to detect very small flaws or defects in the material. Correspondingly, the small coil is very sensitive to the operating environment of the inspection equipment. For example, the probe must remain in contact with the component or specimen being inspected without any gaps therebetween which would cause false readings.
The face of the coil should be oriented substantially normal or perpendicular to the surface of the specimen for maximizing eddy current inspection performance. And, the contact pressure between the probe and the specimen should remain substantially constant as the probe slides along the specimen in order to maintain integrity of the eddy current signal to prevent lift-off of the probe from the specimen which would interrupt that signal.
Although eddy current inspection may be done manually by hand movement of the probe, automated movement of the probe is desired for ensuring accurate inspection and reducing cost for repetitive inspections of multiple features in various components. However, since the target region of a typical specimen has a changing contour subject manufacturing tolerances it is quite difficult to accurately align the probe and automate the inspection process.
For example, even a simple cylindrical hole has a continuously varying surface around the perimeter thereof which correspondingly requires continuous adjustment of the orientation of the eddy current probe. More complex specimen targets include elliptical holes, as well as serpentine features commonly found in gas turbine engines.
For example, each rotor blade in the engine typically includes a mounting dovetail having serpentine dovetail lobes which may be inspected. The dovetails mount in complementary dovetail slots in the perimeter of rotor disks, which slots may require inspection. And, compressor disks may be joined together at curvic couplings including an annulus of scalloped projections for transmitting torque between the disks, which scallops may also be inspected.
In automating eddy current inspection of these typical gas turbine engine components, a conventional multiaxis computer numerically controlled (CNC) machine may be used for mounting the component specimen and the eddy current probe for relative movement. The typical CNC machine includes three translation axes (X,Y,Z) and one or more rotary axis corresponding with the translation axes. In this way, an EC probe may be mounted in the spindle of the machine for automated translation in the three translation axes, with suitable rotation thereof for positioning the probe tip and coil thereat substantially normal to the target surface of the specimen for eddy current inspection thereof.
However, in order to automate the travel of the probe over the changing surface of the target, the probe must be accurately aligned in the machine relative to the specific component specimen mounted therein. The CNC machine has a memory in which the three dimensional (3D) numerical model of the specimen, as represented by its coordinate drawing is stored, with the machine being programmed to follow the stored model in the particular location of the target region thereof being inspected.
Alignment of the probe and component specimen has been a complex and lengthy process in which a specifically configured template is required. For example, the eddy current inspection of an exemplary oil drain hole in a compressor rotor disk has been conducted for many years in this country for production components sold to and used by customers in this and foreign countries. The drain hole has an elliptical profile in an exemplary embodiment, and a specifically configured template is mounted near the drain hole for permitting initial alignment of the probe with a reference aperture in the template. In this way, a stored model of the drain hole using the coordinate system of the machine may be matched to the template for identifying the actual location of the drain hole mounted in the machine.
The conventional alignment process using the template requires a few hours to complete and is repeated multiple times to attach and detach the template until suitable alignment is achieved. The template may then be removed, and the probe automatically driven by the machine to inspect the inner surface of the drain hole around its perimeter at various depths therein using the stored model of the drain hole for numerically guiding the probe.
Accordingly, it is desired to provide an improved method of eddy current inspection of a specimen target which obviates the need for the alignment template.